This invention relates to high solids polymer resin coating compositions containing cross-linking agents and, particularly, to such compositions containing alkylated melamine formaldehyde resin cross-linking agents.
High solids polymer resin coating compositions cross-linked with alkylated melamine formaldehyde resins are finding increasing use for general industrial and appliance applications and as automotive top finishes. Such coating compositions, compared to lower solids coatings, often reduce solvent emission and are more acceptable from an environmental standpoint. In order to achieve higher application solids, lower molecular weight backbone polymers and cross-linking agents are used. In industrial applications where coatings are cured by baking, monomeric hexa(methoxymethyl) melamine resins have achieved widespread acceptance as cross-linking agents.
While it has been found that the higher cross-link density achieved with the chemically efficient hexa(methoxymethyl) melamine resin results in highly resistant films and finishes, it has also been observed that recoating of a baked coating with the same coating system is very difficult. That is, without any sanding, etching or other treatment of the already coated part, no effective intercoat adhesion is achieved. Yet, sanding or etching of the coated part prior to recoating is an expensive and time consuming task. On complicated parts, it is difficult to achieve uniformity with such treatments.
Nevertheless, in many coating operations, recoating is necessary for various reasons. For example, coated parts may be damaged during assembly or multiple coatings, as in the case of two-tone colors on an automobile, may be desired. In a multiple coating operation such as two-tone painting of an automobile, the automobile is first coated with one color which is subsequently baked, the area not to be painted with the second color is masked and then the second color is applied on the remaining uncovered areas of the automobile and cured. Although some high solids coating compositions cross-linked with hexa(methoxymethyl) melamine can function satisfactory in such a recoat operation under controlled laboratory baked conditions, in actual operations they often fail because of decreased control of the curing cycle. It has been found that baking of the first coat at higher temperature or longer time periods during production than should normally occur reduces intercoat adhesion of the second coat.
Thus, it is desirable to have a highly cross-linked coating composition of high solids content which possesses the advantages achieved with hexa(methoxymethyl) melamine as a cross-linking agent but which does not suffer from the disadvantages associated with such a composition such as poor intercoat adhesion. Although mixed methylated n-butylated melamine formaldehyde resin cross-linking agents have improved intercoated adhesion between two coats compared to hexa(methoxymethyl) melamine cross-linked systems in some instances, in many applications the improvement remains insufficient.